Organic pigments, as opposed to dyes, are insoluble in the substrate into which they are incorporated. As a result, the coloristic properties of pigments depend on more that just the chemical formula of the pigment. Other features, such as the crystal form and the size and shape of the pigment particle also impact coloristic properties. Therefore, preparing a pigment involves more than just chemical synthesis; the treatment of the pigment after isolation from the reaction mixture can have a profound impact on how the pigment colors a substrate.
It is well known in the art that organic pigments, such as quinacridones, DPPs, perylenes etc, must be further processed after synthesis to develop the requisite pigmentary properties such as particle size, particle shape, polymorphic phase, tinctorial strength etc. Consequently, crude organic pigments undergo one or more finishing or conditioning steps that typically involve particle size reduction such as wet or dry milling, also referred to as grinding. See, for example R. B. McKay, “Control of the Application Performance of Classical Organic Pigments” in JOCCA, 89 93.
Many variations of milling a pigment including wet milling, dry milling, high speed milling etc, are well known. The procedure is typically carried out in the presence of auxiliaries, such as salts or metal oxides, fillers, rheology control agents and other adjuvants.
U.S. Pat. No. 5,667,580, incorporated herein in its entirety by reference, discloses the preparation of organic pigments with small particle size by milling, blending or pulverizing larger particle size pigments in the presence of fillers including alumina and silica fillers.
U.S. Pat. No. 7,122,081, incorporated herein in its entirety by reference, discloses a process for reducing particle size of organic pigments by milling the crude pigment in the presence of a water soluble styrene copolymer dispersant, optionally a defoamer and/or additive in water.
Optical properties beyond hue and color strength are also important in pigmented plastics and coatings. Effect pigments, also known as gloss or lustrous pigments, are well known as pigments which produce unique coloristic effects. For example, goniochromatic luster pigments exhibit an angle-dependent color change. That is, these pigments produce a “flop effect” whereby the coloristic characteristics change depending on the viewing angle. In general, when a change in viewing angle results in a change in lightness, the effect is referred to as “lightness flop”, and when the change is in hue, the effect is referred to as “color flop”.
Due to their unique color characteristics, the market for effect pigments is growing in such uses as cosmetics, inks, for example security printing inks, plastics and paints, for example, automotive coatings.
Effect pigments are predominately mica pigments, optically variable interference pigments or metallic pigments whose optical effect is based on the directed reflection of light at predominantly sheetlike, mutually parallel-oriented, metallic or strongly refractive pigment particles. These pigments typically have a light-reflecting platelet-shaped core which is coated with alternating low and high refractive layers.
Organic pigments are often used in combination with non-organic effect pigments to obtain a particular color and optical effect. It is known that organic pigments can be prepared which exhibit substantially greater color flop when used in polymer systems.
U.S. Pat. No. 5,618,343, incorporated herein in its entirety by reference, discloses pigment compositions with an unusual flop-effect obtained by incorporating small particle size pigments modified with minute amounts of copper phthalocyanine sulfonic acid.
US Pat. Appl. Pub. No. 2004/0194666, incorporated herein in its entirety by reference, discloses surface modification of transparent organic pigments with a copper phthalocyanine green pigment to produce pigments capable of creating flop-effect, pigment compositions.
U.S. Pat. No. 5,997,627, incorporated herein in its entirety by reference, discloses pigment compositions containing a small particle size porous filler and a pigment, and their use for preparing effect coating compositions exhibiting strong lightness and color flop with a pronounced three dimensional effect.
Although milling or grinding an organic pigment with a mica or other silicate additive to generate a pigment with good chroma is known, it has been found that using particular nano-sized particles of certain silicas, that is fumed silica, as an additive in a controlled milling or grinding process yields organic pigments with not only excellent chroma and dispersion characteristics, but also greatly enhanced color flop, especially when the processed pigment is incorporated into a polymer composition, for example, an automotive coating.